Study reclassifies suspected black hole as rapidly spinning star

On Oct. 27, a new study led by Rhys Seeburger of Liverpool John Moores University reclassified a suspected stellar-mass black hole in the NGC 1850 cluster of the Large Magellanic Cloud as a rapidly spinning star. The paper, titled “Revealing the companion of NGC 1850-BH1,” appeared on arXiv on Oct. 27 and carries DOI 10.48550/arxiv.2610.37262. The work revisits a binary system in the NGC 1850 cluster that astronomers once thought hosted a dormant black hole, using fresh spectroscopic data to re-examine the interpretation.

Earlier work in 2022 identified the system in the NGC 1850 cluster as a 5-solar-mass star paired with an unseen 11-solar-mass dormant black hole, with the normal star outshining its dark companion. That interpretation treated one object as unseen, which motivated the black hole hypothesis. Subsequent studies moved away from that idea but failed to reach a conclusion, leaving the companion’s nature ambiguous.

Using a technique called spectral disentangling on fresh FLAMES data from the Very Large Telescope, the team separated the combined light into individual spectra, uncovering two bright stars. The primary is a cool, puffed-up, stripped star, while the secondary is more massive, hotter, and spins faster; together they mimic the signal once attributed to a black hole companion. FLAMES is a high-resolution spectrograph, and the new observations probe shorter wavelengths than previous data, which makes a hidden luminous star easier to spot. The analysis yields a 1.7-solar-mass donor star and a 5.2-solar-mass luminous companion, with the masses derived from the same spectral fit that revealed two shining components instead of one visible star and one dark object.

The hotter secondary rotates at about 280 kilometers per second, and the simplest explanation for the configuration is past mass transfer between the stars, with the rapid spin indicating a star rather than a compact remnant.

The researchers conclude that NGC-1850 BH1 hosts no black hole, but rather two bright stars that may have a history of interaction. The team stated, “Spectral disentangling has demonstrated that the system known as NGC 1850 BH1 does in fact not contain a compact companion.” They classify the system as an impostor binary because both components shine brightly, eliminating the need for an invisible compact object.

Dormant black holes emit almost no X-rays, making them difficult to detect; they reveal themselves only through the gravitational pull they exert on a nearby visible star. The Milky Way is expected to contain around 100,000 such systems, yet confirmed examples are rare, with VFTS 243 in the LMC and HD 130298 in the Milky Way among the few verified cases of dormant black holes. The expected number of dormant black holes far exceeds the few confirmed examples. Ordinary star pairs can mimic the signal, especially when mass transfer strips one star and bloats the other, allowing impostor systems to enter the sample.

The mass ratio between the two stars is still uncertain, so the system lacks a complete description, the researchers note. Earlier work measured the companion contributing less than 10% of the light in one wavelength range, whereas the new paper finds 30%–50% in that same range, and the research team cannot explain the discrepancy. The researchers plan to get sharper, cleaner spectra, but NGC 1850-BH1 lies in a crowded stellar region and the two stars are faint.

Better data will require space-based Hubble Space Telescope spectroscopic imaging or ground-based blue-efficient long-slit spectrographs such as X-Shooter and integral-field spectrographs on 8-meter-class telescopes. These instruments represent the current options for follow-up observations. The cluster’s well-known age, composition, and distance make NGC 1850 an ideal laboratory for testing models of binary-star evolution and interaction in a nearby galaxy.